Track number of users invited to Slack

**Is your feature request related to a problem? Please describe.**
For statistical purposes we'd like to know an estimate of the number of users who were invited to a customers' Slack channel.

**Describe the solution you'd like**
A new config property, `slack.invitedUserCount`, an integer.

**Describe alternatives you've considered**
We considered tracking an array of IMS user IDs who were invited to the channel.

**Additional context**
There will be some margin of error for this count, hence use of the word "estimate" above. This is because of the manual process to invite the users, where often non-customer emails are denied an invitation to the Slack channel (eg. gmail addresses, or those of partners).